Monitoring system for cardiac surgical operations with cardiopulmonary bypass

ABSTRACT

A monitoring system for cardiac operations with cardiopulmonary bypass comprising: a processor operatively connected to a heart-lung machine; a pump flow detecting device connected to a pump of the heart-lung machine to continuously measure the pump flow value and send it to the processor; a hematocrit reading device inserted inside the arterial or venous line of the heart-lung machine to continuously measure the blood hematocrit value and to send it to the processor; a data input device to allow the operator to manually input data regarding the arterial oxygen saturation and the arterial oxygen tension; computing means integrated in the processor to compute the oxygen delivery value on the basis of the measured pump flow, the measured hematocrit value, the preset value of arterial oxygen saturation, and the preset value of arterial oxygen tension; and a display connected to the processor to display in real-time the computed oxygen delivery value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/064,510, filed Oct. 28, 2013, which is a continuation of U.S.application Ser. No. 13/705,169, filed on Dec. 5, 2012, now U.S. Pat.No. 8,690,784, which is a continuation of U.S. application Ser. No.12/250,212, filed on Oct. 13, 2008, which is a division of U.S.application Ser. No. 11/432,608, filed on May 11, 2006, now U.S. Pat.No. 7,435,220, which claims foreign priority of Italian PatentApplication No. MI2005000866, filed May 13, 2005, and European PatentApplication No. 05021607.6, filed Oct. 4, 2005, the contents of each ofwhich are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention provides a monitoring system to be applied duringcardiac surgery operations requiring a cardiopulmonary bypass (CPB).

BACKGROUND

During cardiac operations with extracorporeal circulation and CPB, theheart and lung function is artificially replaced by a heart-lungmachine. This device receives the venous blood of the patient anddelivers it (through a roller or centrifugal pump) to an oxygenatorwhich provides the carbon dioxide (CO2) clearance and the oxygen (O2)supply to the patient, by means of a trans-membrane exchange betweenblood and a gas flow (oxygen and air at variable concentrations).

SUMMARY

The adequate pump flow for the patient is usually determined on thebasis of specific tables which take into account the body surface areaand the temperature at which the CPB is performed. The lower thetemperature, the lower is the needed pump flow (because the metabolicneeds are decreased). For instance, in a temperature range between 24and 37° C., the flow may vary between 2.0 and 3.0 L/min/m². The goal ofthe pump flow is to provide to the various organs an oxygen amountadequate to sustain the metabolic needs. This does not depend only onthe pump flow, but also on the blood oxygen content and therefore on thehemoglobin (Hb) concentration. Since during CPB there is ahemodilutional anemic status (sometimes severe, with an Hb concentrationthat falls from the normal 15 g/100 ml to 5-10 g/100 ml), it is clearthat the possibility of providing an insufficient oxygen delivery (DO2)is real and quite frequent.

Recently, many scientific papers have demonstrated that insufficientoxygen delivery, either due to excessive anemia, to a low flow, or both,is responsible for postoperative complications and surgical mortalityincrease (References 1-8).

At present, the only continuous monitoring available of a DO2-derivedparameter during CPB is represented by the continuous venous oxygensaturation (SvO2) monitoring. This monitoring is based on therelationship between the oxygen consumption (VO2), that is a metabolicneeds marker, the blood flow and the peripheral oxygen extraction(artero-venous difference):

VO2=Flow×(a−v O2 difference)

If the pump flow is inadequate to maintain the needed VO2, it isnecessary to extract more oxygen from the periphery, and hence thevenous blood will be less saturated (from the normal 75% the saturationmay decrease to 60% or less).

Presently, three venous oxygen saturation (SvO2) systems arecommercially available: (DataMaster Dideco™; CU Terumo™; O2Sat SpectrumMedical™). With the exception of the last one, all these systems measureother variables (hematocrit, electrolytes, etc.) by means of veryexpensive disposable cells. Therefore, these monitoring systems are notroutinely used, and their application is usually reserved for specific,high-risk categories of patients (i.e., congenital, pediatric cardiacsurgery patients).

Moreover, the SvO2 measurement is not a guarantee of adequate pump flow.As a matter of fact, the SvO2 decreases only for a very profound DO2decrease. This is due to the fact that the SvO2 represents the status ofthe whole venous blood coming from the patient. This blood comes even,and to a great extent, from various organs and districts which, underanesthesia, have a VO2 close to zero (muscles, skin, subcutaneoustissues, etc.) and that therefore contribute to the total venous bloodwith their own venous blood having a SvO2 close to 100%. Therefore, incase of a low perfusion to one or more among the most important organs(kidney, brain, gut and the heart itself), which determines a venousblood desaturation down to 60%, the total mixing may result in a globalSvO2 still in a normal range (70-75%).

The aim of the present invention is to overcome the drawbacks of theprior art and create a monitoring system that provides adequatecontinuous and on-line information about the oxygen delivery (DO2) andcarbon dioxide production (VCO2) values during the operation, and aboutthe adequacy of the oxygen delivery (DO2) with respect to the metabolicneeds of the patient.

Another aim of the present invention is to provide a monitoring systemwhich is reliable, accurate, and at the same time cheap and easy to beapplied.

These aims are achieved according to the invention by means of thedevice and method whose characteristics are respectively listed in theattached independent claims. Advantageous embodiments of the inventionare apparent from the dependent claims.

The pathophysiological bases used for structuring the monitoring systemaccording to the invention are the following:

Definitions and Abbreviations

-   -   HCT: hematocrit (%)    -   Hb: hemoglobin (g/dL)    -   CPB: cardiopulmonary bypass    -   T: temperature (° C.)    -   VO2=oxygen consumption (mL/min)    -   VO2i=oxygen consumption indexed (mL/min/m²)    -   DO2=oxygen delivery (mL/min)    -   DO2i=oxygen delivery indexed (mL/min/m²)    -   O2 ER=oxygen extraction rate (%)    -   VCO2=carbon dioxide production (mL/min)    -   VCO2i=carbon dioxide production indexed (mL/min/m²)    -   Ve=ventilation (L/min)    -   eCO2=exhaled carbon dioxide (mmHg)    -   AT=anaerobic threshold    -   LAC=lactates    -   Qc=cardiac output (mL/min)    -   IC=cardiac index (Qc/m²), (mL/min/m²)    -   Qp=pump flow (mL/min)    -   IP=pump flow indexed (Qp/m²), (mL/min/m²)    -   CaO2=arterial oxygen content (mL/dL)    -   Cv O2=venous oxygen content (mL/dL)    -   PaO2=arterial oxygen tension (mmHg)    -   PvO2=venous oxygen tension (mmHg)    -   a=arterial    -   v=venous    -   Sat=Hb saturation (%)

The following equations are applied to implement the monitoring systemaccording to the invention:

VO2=Qc×(CaO2−CvO2) in a normal circulation   (1)

VO2=Qp×(CaO2−CvO2) during CPB   (2)

DO2=Qc×CaO2 in a normal circulation   (3)

DO2=Qp×CaO2 during CPB   (4)

O2 ER=VO2/DO2 (%)   (5)

Hb=HCT/3   (6)

CaO2=Hb×1.36×Sat(a)+PaO2×0.003   (7)

CvO2=Hb×1.36×Sat(v)+PvO2×0.003   (8)

VCO2=Ve×eCO2×1.15   (9)

For instance, a normal subject weighing 70 kg has the followingphysiologic parameters:

-   -   Hb=15 g/dL    -   PaO2=90 mmHg    -   Sat(a) Hb=99%=0.99    -   Pv O2=40 mmHg    -   Sat(v) Hb=75%=0.75    -   Qc=5000 mL/min    -   CaO2=15×1.36×0.99+90×0.003=20.4 mL/dL    -   CvO2=15×1.36×0.75+40×0.003=15.4 mL/dL    -   VO2=5 L/min×(20.4 mL/dL−15.4 mL/dL)=5 L/min×5 mL/dL=5 L/min×50        mL/L=250 mL/min    -   DO2=5 L/min×20.4 mL/dL=5 L/min×200 mL/L=1000 mL/min    -   O2 ER=250/1000=25%    -   VCO2=200 mL/min    -   LAC=<1.5 mMol/L

The oxygen consumption (VO2) represents the metabolic needs of the wholeorganism; it is the sum of the metabolic needs of each specific organ.Under basal conditions (at rest), it is about 3-4 mL/min/kg, i.e. about250 ml/min for a subject weighting 70 kgs. Applying the equations (3)and (7), the oxygen delivery (DO2) may be calculated, and is about 1000mL/min. Therefore, a considerable functional reserve exists, since theDO2 is about 4 times greater than the VO2. The VO2 may increasedepending on the metabolic needs (basically under physical exercise, buteven in pathologic conditions like septic shock). A top level enduranceathlete may reach a maximal VO2 of about 5,000 mL/min.

Of course, to meet these increasing oxygen demands, the DO2 mustincrease as well: it can reach, in an athlete during exercise, the valueof 6,000 mL/min (Qc: 30 L/min with an unchanged arterial oxygen contentof 20 mL/dL). As a consequence, the O2 ER may increase up to 75%.

FIG. 1 is a diagram showing the relationship between DO2 and VO2 in anathlete during physical exercise. If the athlete (that, for example, isrunning a marathon) falls into the dark triangular zone (where the DO2is unable to support the VO2), he is forced to use other metabolicmechanisms in order to develop mechanical energy. As a matter of fact,he will use the anaerobic lactacid metabolism, which develops energy butat the expenses of lactic acid formation, local and systemic acidosis,and finally exercise stops usually within 2 minutes. In other words, theVO2 is physiologically dependent on the DO2.

In the medical field, of course, the situation is different. The DO2 maypathologically decrease in case of: decreased arterial oxygen contentdue to anemia; decreased arterial oxygen content due to hypoxia; anddecreased cardiac output.

However, due to the existence of the above-mentioned physiologicalreserve, the VO2 may be maintained even for a DO2 decrease down to about600 mL/min (DO2i 320 mL/min/m²), due to the increased O2 ER.

FIG. 2 is a diagram showing the relationship between DO2 and VO2 in therange observed during medical conditions (i.e. cardiac operation). Belowa DO2 of 600 mL/min, the VO2 starts decreasing. The patient meets,exactly as the athlete, a lactic acidosis, with lactates (LAC)production. In other words, he experiences a shock.

The DO2 level below which the VO2 starts decreasing and becomespathologically dependent on the DO2 is called the critical DO2(DO2_(crit)). Maintaining the DO2 above this threshold is very importantin many pathological conditions, to avoid an acidosis-shock status. TheDO2_(crit) is higher during a septic shock.

Since 1994, in a paper published in Perfusion, Ranucci and coworkersdemonstrated that in a series of 300 consecutive patients that underwentmyocardial revascularization with CPB, the presence of a severehemodilution was an independent risk factor for a postoperative acuterenal failure (ARF). In particular, the cut-off value was identified atan HCT<25%.

Subsequently, other authors have demonstrated that the lowest HCT duringCPB was an independent risk factor for many “adverse outcomes” incardiac surgery. Stafford-Smith and coworkers, in 1998 (Anesth Analg),confirmed the relationship between hemodilution and ARF.

More recently, the lowest hematocrit on CPB has been recognized as anindependent risk factor for postoperative low cardiac output andhospital mortality by Fang and coworkers (Circulation, 1997), and for animpressive series of postoperative adverse events by Habib and coworkersin 2003 (J Thorac Cardiovasc Surg). The relationship betweenhemodilution and ARF has been subsequently confirmed by Swaminathan andcoworkers in 2003 (Ann Thorac Surg), Ranucci and coworkers 2004 and 2005(Ann Thorac Surg) and Karkouti and coworkers in 2005 (J ThoracCardiovasc Surg). The critical HCT value below which the ARF risksignificantly increases is located between 23% and 26%.

Almost all the authors ascribe this relationship to an insufficientoxygen supply (DO2) to the various organs. The kidney, in particular,due to its physiologic condition of hypoxic perfusion, seems to be athigh risk.

Surprisingly, all the studies demonstrating a relationship between HCTand ARF or other organ damages failed to consider that the HCT is onlyone of the two determinants of the DO2 during CPB: the other is the pumpflow (Qp). This would not influence the DO2 if the Qp was a constant,but this is not the case. In all the studies, the pump flow (Qp) variedfrom a Qpi of 2.0 L/min/m² to a Qpi of 3.0 L/min/m², and the variationwas dependent on the perfusion pressure. An HCT of 24% results in a DO2iof 230 ml/min/m² if the Qpi is 2.0 L/min/m², and of 344 ml/min/m² if theQpi is 3.0 L/min/m².

In a scientific paper in The Annals of Thoracic Surgery, not publishedat the priority dates of the present application, Ranucci and coworkersactually demonstrated that the DO2i, rather than the HCT, is the bestpredictor of ARF. Moreover, in presence of perioperative bloodtransfusions, the DO2i remains the only determinant of ARF. TheDO2_(crit) identified in this paper is 272 ml/min/m², very close to theone previously defined as the DO2i below which the VO2 becomespathologically dependent on the DO2. In other words, maintaining theDO2i above this threshold allows a decrease in the hypoxic organdysfunction or the elimination of the hypoxic organ dysfunction; inpresence of a low HCT, an adequate increase of the Qp may minimize thedeleterious effects of hypoxemia. As a consequence, a continuousmonitoring of the DO2 is of paramount importance in order to limit thepostoperative complications, namely the renal ones.

Measuring a low HCT has poor clinical value, since the only possible(and arguable) countermeasure is a blood transfusion. On the other hand,the DO2 may be modulated by increasing the pump flow.

The level of DO2_(crit), below which the LAC production begins, isidentified by the concept of “anaerobic threshold” (AT). In athletes, itis the level of expressed mechanical power at which the LAC productionbegins; in a patient, it is the level of DO2_(crit), below which the LACproduction begins.

It has been demonstrated that the LAC value during CPB is predictive forpostoperative complications. The problem is that the LAC value is notavailable on-line, and only some devices (blood gas analyzers) provideit. It is however possible to make an “indirect” assessment of the AT.As a matter of fact, under steady conditions, the VO2/VCO2 ratio is aconstant, while during anaerobic lactacid metabolism the VCO2 increasesmore than the VO2. This happens because the lactic acid undergoes thefollowing transformation:

H LAC+NaHCO3=LAC Na+H2CO3

and the H2CO3 is split into H2O and CO2, with a further CO2 production.

FIG. 3 is a diagram showing the relationship between VO2 and VCO2. Therelationship between VCO2 and LAC production has been demonstrated in 15consecutive patients under CPB, in an experimental trial performed bythe inventor himself. In FIG. 4 the graphical relationship between VCO2and LAC production is reported. From this relationship, it appears thata VCOi value of 60 ml/min/m² is a sensitive predictor of lacticacidosis.

Based on the previous experimental information, according to theinvention, a monitoring system for cardiac operations with CPB has beendeveloped. This monitoring system includes:

-   -   a processor connected to a heart-lung machine;    -   a pump flow reading device, connected to a pump of the        heart-lung machine, to continuously measure the pump flow and to        send the pump flow data to the processor;    -   a hematocrit value reading device, inserted inside the venous or        arterial line, to continuously measure the hematocrit value and        to send it to the processor;    -   a data input device, to allow the operator to manually input the        data regarding the arterial oxygen saturation (Sat(a)) and the        arterial oxygen tension (PaO2); computing means integrated in        the processor, to compute the oxygen delivery    -   (DO2i) value on the basis of the measured pump flow (Qp), the        measured hematocrit (HCT), the preset value of arterial oxygen        saturation (Sat(a)), and the preset value of arterial oxygen        tension (PaO2); and    -   a display connected to the processor, to visualize in real-time        the value of DO2i calculated.

Advantageously, the system may be implemented with a CO2 reading device,to continuously detect the exhaled CO2 (eCO2) at the oxygenator gasescape of the heart-lung machine. The data input device allows theoperator to insert the gas flow value (Ve), the computing means computesthe CO2 production (VCO2i) on the basis of the preset gas flow (Ve)value and the detected exhaled CO2 (eCO2), and the display shows thecalculated value of CO2 production (VCO2i).

In one embodiment, the invention provides a monitoring system furthercomprising: comparing means to compare the above mentioned oxygendelivery (DO2i) value with a threshold value of oxygen delivery(DO2i_(crit)); and an alarm which is triggered when the oxygen delivery(DO2i) value falls below the threshold value of oxygen delivery(DO2i_(crit)). In one embodiment, the threshold value of oxygen delivery(DO2i_(crit)) is preset by the operator at a value of about 270ml/min/m². In one embodiment, the monitoring system further comprises atemperature detecting device able to continuously measure the bodytemperature (T) of the patient and to send the temperature values to theprocessor, to be subsequently displayed by the display. The monitoringsystem can further comprise an oxygen delivery threshold computingmeans, receiving as input the temperature (T) value of the patient asdetected.

In one embodiment of the monitoring system, the processor includes ahemoglobin value computing means able to calculate the hemoglobin (Hb)value from the detected hematocrit (HCT) value, and to send it to theoxygen delivery computing means, to be subsequently displayed in thedisplay. In another embodiment of the monitoring system, the processorincludes a computing means able to calculate the indexed pump flow (Qpi)from the pump flow (Qp) and the value of body surface area (BSA) of thepatient. In an embodiment of the monitoring system, the data inputdevice can allow the operator to manually input data regarding theweight and height of the patient so that the computing means is able tocompute the body surface area (BSA) of the patient. In one embodiment,the pump flow detecting device includes a Doppler reading cell.

The invention provides a monitoring system for cardiac operations withcardiopulmonary bypass comprising: a processor operatively connected toa heart-lung machine; a CO2 detecting device placed at the oxygenator ofthe heart-lung machine to continuously detect the exhaled CO2 (eCO2)value and send it to the processor; a data input device allowing theoperator to manually input data regarding the gas flow (Ve) of theheart-lung machine; computing means able to calculate the CO2 production(VCO2i) from the preset gas flow (Ve) and the exhaled CO2 (eCO2)detected; and a display connected to the processor to display inreal-time the above CO2 production (VCO2i) as calculated. In oneembodiment, the monitoring system further comprises comparison means tocompare the above CO2 production (VCO2i) calculated value with a CO2production threshold value (VCO2i_(crit)); and an alarm that istriggered whenever the CO2 production (VCO2i) value exceeds the CO2production threshold value (VCO2i_(crit)).

The invention provides a method of monitoring a patient's parametersduring a cardiac operation with cardiopulmonary bypass and a heart-lungmachine, the method comprising: continuously detecting the pump flow(Qp) from a pump of the heart-lung machine; continuously detecting thehematocrit (HCT) value from the arterial or venous line of theheart-lung machine; setting an arterial oxygen saturation (Sat(a)) valuederived from the arterial blood of the patient; setting an arterialoxygen tension (PaO2) value derived from the arterial blood of thepatient; computing the oxygen delivery (DO2i) of the patient from thedetected and set values; and displaying the oxygen delivery (DO2i) valueas calculated. In one embodiment the method further comprises: settingan oxygen delivery threshold (DO2i_(crit)) value; comparing thecalculated oxygen delivery (DO2i) value with the oxygen deliverythreshold (DO2i_(crit)) value; and triggering a warning signal wheneverthe calculated oxygen delivery (DO2i) value falls below the set oxygendelivery threshold (DO2i_(crit)) value. In one embodiment, the warningsignal is not triggered when the calculated oxygen delivery (DO2i) valuefalls below the set oxygen delivery threshold (DO2i_(crit)) value for aperiod of time shorter than a preset time period. In one embodiment, themethod further comprises the continuous detection and display of thepatient's temperature (T). In one embodiment, the method comprisescalculating the oxygen delivery threshold (DO2i_(crit)) value from thedetected body temperature (T).

The invention provides a method of monitoring a patient's parametersduring a cardiac operation with cardiopulmonary bypass and a heart-lungmachine, the method comprising: continuously detecting the exhaledcarbon dioxide (eCO2) from the oxygenator of the heart-lung machine;setting a gas flow (Ve) value of the heart-lung machine; computing thecarbon dioxide production (VCO2i) of the patient from the detectedvalues of exhaled carbon dioxide (eCO2) and set values of gas flow (Ve);and displaying the calculated carbon dioxide production (VCO2i) value.In one embodiment, the method further comprises: setting a carbondioxide production threshold (VCO2i_(crit)) value; comparing thecalculated carbon dioxide production (VCO2i) value with the set carbondioxide production threshold (VCO2i_(crit)) value; and triggering awarning signal whenever the calculated carbon dioxide production (VCO2i)value exceeds the set carbon dioxide production threshold (VCO2i_(crit))value.

The invention provides a monitoring system for cardiac surgicaloperations comprising: a processor connected to a heart-lung machine;pump flow reading device connected to a pump of the heart-lung machinefor measuring the value of the pump flow (Qp) and sending it to theprocessor; blood hematocrit (HCT) value reading device connected to theheart-lung machine circuit for measuring the HCT value and sending it tothe processor; CO2 detecting device to detect the exhaled CO2 (eCO2) atthe oxygenator gas escape, data input device to manually input datarelative to the arterial oxygen saturation (Sat(a)), arterial oxygentension (PaO2), and gas flow (Ve); and computing means to compute theoxygen delivery value (DO2i) and the CO2 production value (VCO2i) on thebasis of the detected and set values, and a display connected to theprocessor, to show the calculated values of (DO2i) and (VCO2i).

Further characteristics of the invention will be clarified by thefollowing detailed description, referring to a non-limiting embodimentthereof illustrated in the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between VO2 and DO2 inan athlete under physical exercise.

FIG. 2 is a graph illustrating the relationship between VO2 and DO2 in apatient under cardiac operation.

FIG. 3 is a graph illustrating the relationship between VCO2 and VO2.

FIG. 4 is a graph illustrating the relationship between LAC and VCO2i.

FIG. 5 is a schematic view of a heart-lung machine connected to thepatient and a block diagram of a monitoring system according to theinvention.

FIG. 6 is a block diagram illustrating in a more detailed way amonitoring system according to the invention.

FIG. 7 is a front view of a monitoring device according to theinvention, showing a screen view of the display.

FIG. 8 is a view of a monitoring device according to the inventionsimilar to FIG. 7, in a minimal configuration.

In the following description, and with reference to FIGS. 5, 6, 7, and8, the monitoring system for cardiac operations with cardiopulmonarybypass is described, according to the invention.

FIG. 5 shows a patient 1 laying on a surgical table 2. A heart-lungmachine, comprehensively identified by the number 3, is connected to thepatient 1. As already known in the art, a heart-lung machine 3 comprisesa venous extracorporeal circuit, collecting blood from the venous systemof the patient. A roller or centrifugal mechanical pump 4 pumps thevenous blood from a venous extracorporeal circuit towards an oxygenator5, whose role is removing CO2 from the venous blood and supplying oxygen(O2). The blood oxygenated by the oxygenator 5, is sent, again by thesame roller or centrifugal pump 4, to an arterial extracorporeal circuitconnected to the arterial system of the patient, therefore creating atotal bypass of the heart and lungs of the patient.

DETAILED DESCRIPTION

The monitoring system, according to the invention, comprehensivelyidentified by the reference number 10, is operatively connected to theheart-lung machine 3. The monitoring system 10 comprises a processorable to perform calculations, as subsequently explained, and a monitorscreen or display 11 that works as an interface with the operator.

Manual Data Input from the Operator

Using the knob 50 (FIGS. 7 and 8), placed in the front part of themonitoring system 10, the operator may input data that will be stored bythe processor memory of the monitoring system 10. Data manually insertedby the operator are:

-   -   (1) Height and weight of the patient.    -   (2) The arterial oxygen saturation (Sat(a)). This value, during        CPB procedures, is usually 100%. However, in case of        extraordinary events like an oxygenator malfunction, the value        of (Sat(a)) may decrease. In this case, the operator may        manually change this value using the knob 50.    -   (3) The arterial oxygen tension (PaO2) value. This PaO2 value is        measured by the perfusionist on the arterial blood of the        patient with blood gas analysis, using an adequate and specific        device.    -   (4) The gas flow value (Ve). This Ve value is established by the        perfusionist operating the heart-lung machine 3. Generally, the        Ve is regulated with a flow-meter, in a range between 1 and 5        L/min, according to the patient's parameters. This Ve value        rarely changes during a CPB procedure, and therefore can be        manually inserted by the operator. However, as an alternative,        the monitoring system 10 may include an electronic flow-meter        connected to the heart-lung machine 3, to continuously detect        the Ve value.

Heart-Lung Machine Interfaced Data

The monitoring device 10 is equipped with some electrical connections tothe heart-lung machine 3, as to continuously receive data collected byadequate sensors placed in specific positions of the heart-lung machine.These continuously collected data are:

-   -   (1) Patient's body temperature (T). This temperature T is        continuously measured by a temperature probe inserted inside the        esophagus or the rectum of the patient. The temperature probe 40        sends an electronic signal of the temperature to a monitor of        the heart-lung machine visualizing, in real-time, the        temperature value. In this case, it is sufficient to interface        with an electrical connection the monitor of the heart-lung        machine 3 with the monitoring device 10, for a continuous input        of the temperature value T.    -   (2) Exhaled carbon dioxide (eCO2). This eCO2 value is        continuously measured through a CO2 detector 41 placed at the        gas escape of the oxygenator 5 to detect the sidestream CO2        exhaled from the oxygenator 5. The CO2 detector 41 can be any        kind of CO2 detector among the various commercially available        and re-usable capnographs.    -   (3) Hematocrit (HCT). The HCT value is continuously measured        through a hematocrit reading cell 42 placed inside the arterial        or venous circuit of the heart-lung machine 3. For instance, in        FIG. 5, the hematocrit reading cell 42 is placed inside the        arterial line between the pump 4 and the oxygenator 5. The        hematocrit reading cell 42 is commercially available and        disposable.    -   (4) Pump flow (Qp). The Qp value is continuously measured        through the Doppler reading cell 43, placed on the arterial line        of the heart-lung machine 3. This kind of Doppler reading cell        43 measures the blood flow on the basis of the Doppler principle        (red cells velocity).

If the pump 4 of the extracorporeal circuit is a centrifugal pump, it isalready equipped with the Doppler reading cell 43. Conversely, if thepump 4 is a roller pump, the Doppler reading cell 43 may be added. Inthe alternative, the Doppler reading cell 43 may be omitted, since theroller pump head is provided with a flow measuring system. In this case,the data regarding the pump flow Qp is directly sent to the monitoringdevice 10.

With specific reference to FIG. 6, the operation of the monitoringsystem 10 is described below. The processor of the monitoring system 10includes a first computing program 12 that, based on the weight andheight of the patient as input by the operator calculates, according topre-defined tables, the body surface area (BSA) of the patient.

The BSA value is sent to a second computing program 13 that receives theinput value of the pump flow Qp as detected by the pump 4 of theheart-lung machine 3. The second computing program 13 calculates theindexed pump flow Qpi, according to the relationship QpI=Qp/BSA.

A third computing program 14 receives the input value HCT as detected bythe hematocrit reading cell 43 placed inside the venous or arterial lineof the heart-lung machine. The third computing program 14, based on theequation (6), calculates the hemoglobin value Hb. The Hb value is sentto the display 11 and displayed in a window 51 of the display 11 (FIG.7).

The pump flow indexed Qpi computed by the second computing program 13and the hemoglobin value Hb computed by the third computing program 14are sent to a fourth computing program 15 that receives as input valuesthe values of arterial oxygen saturation (Sat(a)) and arterial oxygentension (PaO2) manually entered by the operator. The fourth computingprogram 15, according to the equation (4), calculates the indexed oxygendelivery value (DO2i).

As shown in FIG. 7, the DO2i value is visualized in real time in awindow 53 of the display 11 and as a graphical pattern 52 (as a functionof time). The display 11 is provided with a chronometer window 56showing the time passed from the beginning of the CPB.

As shown in FIG. 6, the DO2i value is sent to a comparator 18 whichcompares it to a threshold value of DO2i_(crit) that is displayed in awindow 54 (FIG. 7) of the display 11. This threshold value is set at 270ml/min/m² at a temperature between 34° C. and 37° C., and decreases as afunction of temperature, in a linear fashion.

Therefore the threshold value of DO2i_(crit) be preset by the operatoror may be calculated by a computing program 17 depending on thetemperature value T determined by the temperature probe 40. Thetemperature T value determined by the probe 40 is sent to the display 11to be displayed in a window 55.

When the DO2i value falls below the DO2i_(crit), the comparing device 16sends a control signal to an alarm 16 that is triggered, alerting theoperator of a potentially dangerous condition.

The alarm 16 is not triggered by brief decreases of the pump flow Qp(often needed during CPB). Therefore, the alarm 16 could be set to beactivated after 5 minutes of consecutive detection of a DO2i below theDO2i_(crit). However, a recording of all the periods of low flow can bemade, to analyze and avoid the possibility that many short periods oflow flow may create an additional effect. It is reasonable to considerno more than 20 minutes (as a total) of DO2i below the DO2i_(crit)during a normal CPB lasting about 90 minutes. The monitoring device 10is equipped with a computing program 19, which receives as input valuesthe exhaled carbon dioxide eCO2 as detected by the CO2 sensor 41 and thegas flow Ve set by the operator. According to these input data, thecomputing program 19 calculates the indexed carbon dioxide productionVCO2i applying the equation (9).

The VCO2i value as calculated by the computing program 19 is sent to thedisplay 11 and displayed in real time in a window 57 (FIG. 7) in itsgraphical relationship 58 as a function of time.

The VCO2i value is sent to a second comparator 20 which compares it withan anaerobic threshold value VCO2i_(crit) set by the operator; bydefault the VCO2i_(crit) is preset at 60 ml/min/m². As shown in FIG. 7,the display 11 is provided with a window 59 showing the value ofanaerobic threshold VCO2i_(crit) set by the operator.

Back to FIG. 6, when the VCO2i exceeds the VCO2i_(crit) an alarm signalis sent to a second alarm 21, which, when triggered, alerts the operatorof a warning condition. Moreover, as shown in FIG. 7, the display 11 isprovided with: a window 60 where the gas flow value Ve set by theoperator is displayed; a window 61 where the indexed pump flow value Qpiarriving from the computing program 13 is displayed; and a window 62where the body surface area of the patient is displayed as calculated bythe computing program 12.

The monitoring system 10 can be equipped with a data recording systemand a printer interface, and/or a digital data recording system. Thedisplay 11 could include two configurations: a complete configuration,as the one shown in FIG. 7, and a reduced configuration, onlyconsidering the DO2 parameter, as shown in FIG. 8.

The above description and accompanying drawings are provided for thepurpose of describing embodiments of the invention and are not intendedto limit the scope of the invention in any way. It will be apparent tothose skilled in the art that various modifications and variations canbe made in the systems and methods for cardiac operations withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

REFERENCES

-   1. Ranucci M., Pavesi M., Mazza E., et al. Risk factors for renal    dysfunction after coronary surgery: the role of cardiopulmonary    bypass technique. Perfusion 1994; 9:319-26.-   2. Stafford-Smith M., Conlon P. J., White W. D., et al. Low    hematocrit but not perfusion pressure during CPB is predictive for    renal failure following CABG surgery [Abstract]. Anesth Analg 1998;    86: SCA 11-124.-   3. Fang W. C., Helm R. E., Krieger K. H., et al. Impact of minimum    hematocrit during cardiopulmonary bypass on mortality in patients    undergoing coronary artery surgery. Circulation 1997; 96 (suppl    II):194-9.-   4. Habib R. H., Zacharias A., Schwann T. A., Riordan C. J.,    Durham S. J., Shah A. Adverse effects of low hematocrit during    cardiopulmonary bypass in the adult: should current practice be    changed? J Thorac Cardiovasc Surg 2003; 125:1438-50.-   5. Swaminathan M., Phillips-Bute B. G., Conlon P. J., Smith P. K.,    Newman M. F., Stafford-Smith M. The association of lowest hematocrit    during cardiopulmonary bypass with acute renal injury after coronary    artery bypass surgery. Ann Thorac Surg 2003; 76:784-92.-   6. Karkouti K., Beattie W. S., Wijeysundera D. N., et al.    Hemodilution during cardiopulmonary bypass is an independent risk    factor for acute renal failure in adult cardiac surgery. J Thorac    Cardiovasc Surg 2005; 129:391-400.-   7. Ranucci M., Romitti F., Isgr G., et al. Oxygen delivery during    cardiopulmonary bypass and acute renal failure following coronary    operations. Ann Thorac Surg 2005; in press.-   8. Ranucci M., Menicanti L., Frigiola A. Acute renal injury and    lowest hematocrit during cardiopulmonary bypass: not only a matter    of cellular hypoxemia. Ann Thorac Surg 2004; 78:1880-1.-   9. Demers P., Elkouri S., Martineau R., et al. Outcome with high    blood lactate levels during cardiopulmonary bypass in adult cardiac    surgery. Ann Thorac Surg 2000; 70:2082-6.

I claim:
 1. A monitoring system for cardiac operations withcardiopulmonary bypass utilizing a heart lung machine and an oxygenatorfluidly coupled to the heart lung machine, the monitoring systemcomprising: means to obtain an arterial oxygen saturation value; meansto obtain a venous oxygen saturation value; means to obtain a hematocritvalue; a pump flow reading device configured to obtain a pump flow ratefrom a pump of the heart lung machine; a processor operably connected tothe means to obtain an arterial oxygen saturation value, the means toobtain a venous oxygen saturation value and the means to obtain ahematocrit value, the processor configured to calculate an oxygendelivery value and an oxygen consumption value; and a display operablyconnected to the processor, the display configured to display each ofthe arterial oxygen saturation value, the venous oxygen saturationvalue, the hematocrit value, the oxygen delivery value, the oxygenconsumption value and the pump flow rate.
 2. The monitoring system ofclaim 1, wherein the processor is further configured to calculate anindexed oxygen delivery value and display the calculated indexed oxygendelivery value on the display.
 3. The monitoring system of claim 1,wherein the processor is further configured to calculate an indexedoxygen consumption value and display the calculated indexed oxygenconsumption value.
 4. The monitoring system of claim 1, wherein theprocessor is further configured to calculate an oxygen extraction ratioand display the calculated oxygen extraction ratio on the display. 5.The monitoring system of claim 1, wherein the processor is furtherconfigured to calculate a cardiac index value and display the calculatedcardiac index value on the display.
 6. The monitoring system of claim 1,wherein the processor is configured to obtain a hemoglobin value anddisplay the hemoglobin value on the display.
 7. The monitoring system ofclaim 1, wherein the processor is configured to calculate a body surfacearea value and to display the calculated body surface area value on thedisplay.
 8. A method of monitoring a patient during a cardiac operationwith cardiopulmonary bypass and a heart lung machine operably connectedto an oxygenator configured for oxygenating blood, the methodcomprising: obtaining an arterial oxygen saturation value; obtaining avenous oxygen saturation value; obtaining a hematocrit value;calculating an oxygen delivery value; calculating an oxygen consumptionvalue; and displaying each of the arterial oxygen saturation value, thevenous oxygen saturation value, the hematocrit value, the oxygendelivery value and the oxygen consumption value.
 9. The method of claim8, further comprising: calculating an indexed oxygen delivery value; anddisplaying the calculated indexed oxygen delivery value.
 10. The methodof claim 8, further comprising: calculating an indexed oxygenconsumption value; and displaying the calculated indexed oxygenconsumption value.
 11. The method of claim 8, further comprising:calculating an oxygen extraction ratio; and displaying the calculatedoxygen extraction ratio.
 12. The method of claim 8, further comprising:calculating a cardiac index value; and displaying the calculated cardiacindex value.
 13. The method of claim 8, further comprising: obtaining apump flow rate; and displaying the obtained pump flow rate.
 14. Themethod of claim 8, further comprising: obtaining a hemoglobin value; anddisplaying the hemoglobin value.
 15. The method of claim 8, furthercomprising: calculating a body surface area value; and displaying thecalculated body surface area value.